The invention relates to a process and apparatus for the separation of higher-boiling hydrocarbons from a gaseous stream containing the latter and lower-boiling components.
Processes are known wherein the gaseous stream is cooled, partially condensed, and separated into a liquid fraction and a gaseous fraction. The liquid fraction is fractionated by rectification into a product stream containing lower-boiling components. The gaseous fraction separated after partial condensation is conducted to a scrubbing column wherein higher hydrocarbons are scrubbed out.
Such processes serve above all for the separation of higher-boiling hydrocarbons from natural gases or other gases, e.g., refinery waste gases or residual gases from a synthesis process. Since the market prices for C.sub.3 /C.sub.4 hydrocarbon mixtures have risen, increased efforts have been directed to provide improved processes for the production thereof.
DOS 3,511,636 discloses a process of the type discussed above relating to the separation of C.sub.2+ or Cp.sub.3+ hydrocarbons from a gaseous stream containing light hydrocarbons and, in some cases, components boiling lower than methane. The gaseous stream to be fractionated is partially condensed into a gaseous stream essentially free of C.sub.2+ or C.sub.3+ hydrocarbons and into a liquid stream containing essentially only C.sub.2+ or C.sub.3+ hydrocarbons.
The liquid stream is freed from the remaining lower-boiling components in a rectifying column, together with a further fraction. A high-purity C.sub.2+ or C.sub.3+ hydrocarbon fraction is withdrawn from the bottom of the column and can be passed onto further fractionating.
The gaseous stream of the partial condensation is conducted into a scrubbing column in order to remove higher-boiling components that have remained therein. For this purpose the residual gas from the head of the rectifying column is introduced, in partially condensed form, as scrubbing medium into the scrubbing column wherein the higher-boiling components are scrubbed out of the rising gaseous stream. From the bottom of the scrubbing column, a fraction is withdrawn enriched in higher-boiling components and is fed, as the above-mentioned further fraction, into the rectifying column for separation. At the head of the scrubbing column, a gaseous fraction depleted in higher-boiling hydrocarbons is withdrawn.
Although the yield of C.sub.2+ and C.sub.3+ hydrocarbons is satisfactory, a valuable amount of higher-boiling components still escapes together with the gaseous fraction from the scrubbing column. In order to increase the yield further, the temperature or the pressure of the scrubbing column could be lowered. However, for this purpose an expansion machine such as a turbine would be required, for example, which represents an expensive part of the apparatus.
Under certain conditions, especially in the case of a high proportion of CO.sub.2 in the gaseous stream to be separated, lowering of the temperature of the scrubbing column is disadvantageous. In the event of a high proportion of CO.sub.2, lowering the temperature of the scrubbing column can lead to CO.sub.2 freezing out as a solid. This would adversely affect the course of the process.
Therefore, losses in yield had to be tolerated heretofore in the conductance of the above-mentioned process, if the CO.sub.2 should not, or could not be removed from the gaseous stream by expensive preliminary purification steps.